1. Field of the Invention
The present invention relates to an optical information reader having a function for adjusting a focal length of an imaging lens and a method of reading optical information.
2. Description of the Related Art
A conventional handheld optical information reader for reading optical information, such as a barcode, a two-dimensional code, or other similar codes, is installed in a handheld body case. In particular, the optical information reader has a photodetector, an imaging unit with an imaging lens, and a reading unit composed of a lighting device, which are installed in the body case, respectively.
A light is supplied from the lighting device through a reading window formed at one end portion, that is, a head portion, of the body case. The light is irradiated to a reading target, such as a barcode. A light reflected from the reading target according to the irradiated light is entered through the reading window into the imaging unit. The light entered into the imaging unit is focused on the photodetector by the imaging lens to be imaged thereon, so that an image corresponding to the reading target is picked up by the photodetector.
In this case, a reading distance defined as a distance between the reading window and the reading target is previously determined depending on an optical characteristic, such as a focal length of the imaging lens. A user, therefore, handles the body case of the optical information reader to locate it so that the reading window of the body case is positioned at the reading distance from the reading target or adjacent to the reading distance and the reading window is opposite to the reading target. After the positioning of the reading window of the body case of the optical information reader, the reading operations of the reading target set forth above are started.
In addition to the structure, an optical information reader having a mechanism capable of adjusting the focal length of the imaging lens has been created. The mechanism expands the range of usable reading distances of the optical information reader, thereby improving convenience of the users.
One example of the optical information reader with the focal length adjusting mechanism is disclosed in U.S. Pat. Publication No. 6,066,857 corresponding to Japanese Unexamined Patent Publication No. 2003-507778.
In the disclosed optical information reader, a focusing disk is rotatably disposed between an objective lens and a two-dimensional photodetector, such as CCD (Charge Coupled Device), so that an axis of the rotation of the focusing disk is offset with respect to an optical axis of the objective lens.
The focusing disk is provided at its circumferential edge with a plurality of more or less raised optical shims that have different thicknesses suitable to focus light from the objective lens on the CCD. Rotation of the focusing disk allows the focal length of the objective lens to be adjusted.
Another one example of the optical information reader with the focal length adjusting mechanism is disclosed in Japanese Unexamined Patent Publication No. H10-247215.
In the disclosed optical information reader, an optical path length variable member is disposed between an imaging lens and a CCD. The optical path length variable member has a pair of transparent glass plates that are arranged to be opposite to each other along a lens' optical axis at a predetermined interval. The optical path length variable member also has a cylindrical bellows disposed between the paired transparent glass plates to be expandable in the optical axis, and an optical refraction fluid filled in the bellows.
The optical path length variable member has a driving mechanism composed of an annular magnet mounted on one of the paired transparent glass plates to surround the bellows. The optical path length variable member also has a pair of coils disposed to the other of the paired transparent plates to surround the bellows and the annular magnet.
When a driving current is supplied to flow in the coils, the driving current generates magnetic force so that the magnetic force causes the paired transparent glass plates to relatively move in parallel to the optical axis. The relative movements of the paired transparent glass plates permit the length of the optical refraction fluid in the optical axis to vary, thereby adjusting the optical path length between the imaging lens and the CCD.
The former optical information reader disclosed in U.S. Pat. Publication No. 6,066,857, however, requires the plurality of optical parts, such as the optical shims having different thicknesses corresponding to the focal lengths, respectively, which may cause the number of optical parts and the whole size of the optical information reader to increase.
Especially, when using a one-dimensional photodetector as the photodetector, whose light-receptive area is long in one-dimensional direction, each of the optical parts (optical shims) is large in size, which may cause the whole size of the optical information reader to more increase.
On the other hand, in the later optical information reader disclosed in Japanese Unexamined Patent Publication No. H10-247215, the optical path length variable member requires keeping the driving current flowing in the coils to maintain the positions of the paired glass plates, causing electric power consumption of the optical information reader to increase. This results in that the optical path length variable member may be unsuitable for battery-drive handheld optical information readers.